Method for producing an absorber for a solar collector, and a solar collector

ABSTRACT

A method for producing a solar heat absorber for a solar collector, the solar heat absorber having a fluid inlet and a fluid outlet. The method includes the steps of providing a first and a second substantially flat metal plate, laser welding the plates together at least along a perimeter of the plates, providing at least a circumferential weld while pressing the plates together, placing the welded plates in a press that provides its pressure at the weld or welds and the press applying a pressure of at least 200 tons per m 2  of absorber, and then providing a fluid under a pressure to the absorber via at least one of the fluid inlet and the fluid outlet.

FIELD OF THE INVENTION

The invention relates to a method for producing an absorber for a solar collector, and a solar collector provided with such a solar heat absorber.

BACKGROUND OF THE INVENTION

Solar collectors as such are known in the art. Conventionally, conduits are provided in a serpentining manner and exposed to solar radiation. A fluid is led though the conduit in order to take up solar heat. These known assemblies are difficult to produce in mass production, and have a relatively low output.

In EP1797373B1, a solar collector is disclosed that comprises a first substantially flat front plate for collecting solar energy, a second substantially flat back plate which is fixed at least substantially along the edges on the underside of the first plate, a space between the two plates, an inlet for feeding a liquid to the space, an outlet for discharging the liquid from the space, whereby the liquid can flow from the inlet to the outlet, wherein the space between the plates is formed in that a profile defining a substantially flat, zigzag form of the space is arranged in at least one of the plates, and the solar collector comprising turbulence members for causing turbulence in the liquid in the space, which turbulence members partly close the through flow channel in vertical direction.

Production of such a solar collector in mass production proved difficult.

SUMMARY OF THE INVENTION

The invention aims to provide method for producing a solar collector that can be produced in mass production. The invention alternatively or in particular seeks to provide a method for producing a solar collector that can be produced in a reliable manner.

To that end, the invention provides a method for producing an absorber for a solar collector, the solar heat absorber having a fluid inlet and a fluid outlet, the method comprising the steps of: providing a first and a second substantially flat metal plate; laser welding the plates together at least along a perimeter of the plates, providing at least a circumferential weld, while pressing the plates together; placing the welded plates in a press that provides its pressure at the weld or welds and the press applying a pressure of at least 200 tons per m² of absorber; providing a fluid under a pressure to the absorber via at least one of the fluid inlet and the fluid outlet, wherein the fluid is applied at a pressure difference between the fluid inlet and the fluid outlet inflating the absorber, wherein the pressure difference is at least 20 bar, expanding the welded metal plates for providing a permanent fluid channel between the plates and fluid-coupling the fluid inlet to the fluid outlet.

It was found that providing a leak-tight absorber for a solar collector was difficult, and that first attaching the mainly flat plates to one another before providing the channels resulted in a sure and reliable mass production process that allows high speed production of the absorbers for the solar panels. In particular, the production allows including an easy leak-test.

In this document, circumferential is not limited to a circular shape, but is intended to refer to also include general shapes including rectangular.

In particular, using laser welding technique, a continuous lending seam or fillet can be created at the perimeter of the metal plates. Thus, a fluid-tight coupling of the metal plates can be provided. In an embodiment, the laser welding provides spots, in particular elongated spots, that connect or partially overlap. By providing a partial overlap, a fluid-tight welding seam can be assured.

The solar collector further can comprise a housing for the solar heat absorber. An embodiment of such a housing is in general disclosed in EP1797373B1, incorporated by reference as is fully set forth in this text.

In particular, in an absorber for a solar collector, the temperature cycles are very specific and place a high demand on the quality of welding and other provisions in order to prevent for instance leakage. In a solar collector, as soon as the sun shines bright, the temperature can easily rise up to almost 100 degrees Celsius in a short time, even within minutes. As soon as a cloud comes before the sun, or in winter rain or hail comes down, the temperature can drop within an hour to below zero degrees. Furthermore, during 24 hours the temperature can go through many cycles. Thus, the thermal stress load upon welds en fluid inlets is considerable.

The currently-proposed production method further allows a relatively stiff absorber to be produced. In fact, this again allows the thickness of the metal plates to be reduced. In fact, it is even considered to use for instance RVS plates with a thickness between 0.3 and 0.8 mm. This allows a swift and lossless heat transfer, less material use and a lighter construction.

In an embodiment, the method further comprises providing at least one through holes in one of the plates for providing the fluid inlet, and at least one through hole in one of the plates for providing the fluid outlet.

In an embodiment, the method comprising pressing the plates on top of one another along at least a perimeter of the plates.

In an embodiment, the press comprises a die having a pattern which has the shape of the weld and which extends 1-20 mm from the basic surface of the die, in an embodiment extending 1-5 mm from the basic surface.

In an embodiment, the press comprises a first die having the pattern the shape of the weld, and a second, opposite die that also has the shape of the weld such that when pressing the welded metal plates, both patterns press the weld between them.

In an embodiment, the welding of the metal plates includes proving spot welds in the fluid channel for welding together the metal plates in the fluid channel for providing flow turbulence means between the inlet and the outlet.

In an embodiment, the weld comprises a circumferential weld enclosing at least the inlet and the outlet, and providing one or more welds that provide boundaries of one or more zigzag fluid channels fluid coupling the inlet and the outlet, in an embodiment providing in an alternating manner first and second welds with first welds that run from the circumferential weld at one first end of the absorber end and that end before the circumferential weld at an opposite second end of the absorber and second welds that run from the circumferential weld at the opposite, second end and that end before the circumferential weld at the first end of the solar heat absorber.

In an embodiment, spot welds are welded within a circumference of the circumferential weld.

In an embodiment, the circumferential weld is provided to enclose at least the fluid inlet and the fluid outlet, and one or more welds are provided that provide boundaries of one or more fluid channels between the fluid inlet and the fluid outlet, in particular the welds are provided to provide a fluid channel spiralling from a side of the absorber to a centre of the absorber.

In an embodiment, before the plates are placed on top of one another, weld-preventing layers are provided between the metal plates at the positions of the fluid inlet and the fluid outlet.

In an embodiment, at the location of an inlet or an outlet, one of the metal plates of the absorber is provided with a through hole.

In an embodiment, before the metal plates are expanded, nipples provided with a flange are provided at the through holes and welded to the metal plate with the flange around the through holes before the fluid under pressure is provided to the absorber.

In an embodiment, before the metal plates are expanded, nipples are provided at the through holes and attached to the metal plate and around the through holes before the fluid under pressure is provided to the absorber without attaching together the metal plates at or near the locations of the through holes.

The invention further relates to a solar collector comprising a solar heat absorber produced using the method described in this application.

The invention further pertains to a solar collector comprising a solar heat absorber comprising a first and second metal plate welded together on top of one another at their perimeter via a continuous, circumferential weld, a fluid inlet and a fluid outlet within the circumferential weld, at least one continuous weld extending between the fluid inlet and the fluid outlet, and a plurality of spot welds within the circumferential weld.

In the current absorber, the welding is done via laser welding. In this process, subsequent spots are welded. An alternative welding technology that might be considered for producing the continuous, leak-tight welds would be seam welding, which is a contact-based welding technique that would require a lot of manipulation of the plates during welding. Laser welding, in contrast, is contactless and does not require the plates to be manipulated during welding.

The laser welds are produced using a fiber laser with a very high yield of 25-30%. The type of weld that is produced is a deep weld. This means that the melting zone of the weld during welding can extend up to 80% of the lower plate.

The turbulence members can have a spot diameter of 3-15 mm, in most applications, the turbulence members have an substantially circular spot with a diameter of between 4 and 10 min.

The terms “upstream” and “downstream” relate to an arrangement of items or features relative to the flow of a (heat absorbing) fluid from a tank and through the solar collector.

The term “substantially” herein, such as in “substantially consists”, will be understood by the person skilled in the art. The term “substantially” may also include embodiments with “entirely,” “completely,” “all,” etc. Hence, in embodiments the adjective substantially may also be removed. Where applicable, the ter “substantially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%.

The term “comprise” includes also embodiments wherein the term “comprises” means “consists of”

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

The devices or apparatus herein are amongst others described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation or devices in operation.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “to comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device or apparatus claims enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The invention further applies to an apparatus or device comprising one or more of the characterising features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterising features described in the description and/or shown in the attached drawings.

The various aspects discussed in this patent can be combined in order to provide additional advantages. Furthermore, some of the features can form the basis for one or more divisional applications.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which is shown:

FIGS. 1-7 illustrates various steps in a method for producing an absorber for a solar collector.

FIG. 8 illustrate a transport band for transporting absorbers.

FIGS. 9 and 10 illustrates a nipple for an inlet and/or an outlet of an absorber for a solar collector.

FIGS. 11-13 illustrates some embodiments of the fluid channels of the solar-heat absorber.

DETAILED DESCRIPTION OF EMBODIMENTS

In FIGS. 1-7, various steps in a method for producing an absorber for a solar collector. In the step of FIG. 1, two metal plates 1 are already welded together at least along their perimeter, to form a pre-shape for an absorber. Furthermore, an inlet and an outlet have already been provided on the surface opposite of the surface that is visible. In FIG. 1, a press 2 with two dies 3, 4 that are currently spaced apart are ready to receive the welded metal plates 1. The lower die 4 is provide with two slots 5, 7 that can allow the inlet and outlet of the of the absorber to remain available. Two adapters 6, 8 are provided that can attach to the inlet and outlet of the absorber. In this embodiment, adapter 6 is attached to a source of fluid under pressure, and adapter 8 is here provided with a stop 9.

In FIG. 2, the two welded plates 1 that will become the absorber are placed into the press and with the inlet and outlet, that are provided in the lower of the two plates 1 that will make up the absorber, in the slots 5 and 7.

In FIG. 3, the two welded plates 1 rest on the lower die 4 and the adapters 6 and 8 are coupled to the inlet and the outlet via slots 5 and 7.

In FIG. 4, the upper die 3 is now lowered onto the two welded plates. The dies 3, 4 are now pressed together. As soon as the required pressure is attained, a fluid pressure is applied via adapter 6. In an alternative, it is also possible to apply pressure via both adapters 6, 8. Here, water is applied at a pressure of about 20-60 bar, depending on the thickness of the metal plates. In an example, metal plates of stainless steel with a thickness of 0.2-1.0 mm are used, and a pressure of 30-50 bar is applied. In particular, metal plates with a thickness of 0.3-0.8 are used. The pressure is sufficient to deform the plates and to provide for instance a serpentining fluid channel running from the inlet to the outlet. The press 2 defines the height of the fluid channel and prevents rupture of the welds. This will be explained below.

In FIG. 5, the press 2 is opened and the absorber with serpentining or zigzag fluid channel can be seen. This shows the circumferential weld and transverse welds that extend from the circumferential weld and that extend almost to the opposite side. Alternatingly, a transverse weld starts from one side and from the opposite side.

In FIG. 6, the press is opened and the adapters 6, 8 are uncoupled from the inlet and the outlet. In this drawing, on the lower die 4 a provision can be seen that controls the expanding of the absorber: The die has the pattern of the welded parts extending 2-10 mm from the further surface 11 of the die. In the current embodiment, the upper die 3 has the same pattern in mirror image. In this way, when the dies are pressed together with an unexpanded absorber pressed between then, at first pressure is exerted on the welded parts. The remaining parts of the yet unexpanded absorber has room to expand. If the fluid is now introduced, it usually has a pressure that is sufficient to blow up the yet unexpanded absorber, but it is less than the pressure per unit surface area of the press. In this way, the force exerted by the fluid that is used to blow up the absorber is not applied to the welds. Furthermore, the remaining surface 11 of a die between the extending weld-pattern 11 determines the shape of the expanding of the absorber. Thus, the remaining surface 11 can be flat, or it can have a profile that can be transferred onto the absorber 1 when it is being expanded. Thus, for instance the pattern of FIGS. 5-8 can be produced on the absorbers.

In FIG. 7, the expanded absorber 1 is removed from the press 2 for a next production step. A next production step may comprise provided a solar radiation absorption layer. Usually, the metal surface is coated using a paint, often matt black, that absorbs as much as possible of the solar radiation. This may also includes absorbing infrared radiation and/or Ultraviolet radiation.

In FIG. 8, a transport device 20 is shown that has a transport band 21 that is adapted for transportation of expanded and coated absorbers. The transport band 20 has supporting provisions 22 for supporting absorbers. These supporting provisions are here plates 22 that are interspaced and extend from the transport band 21. At the turns of the band 21, in this way a nip 23 is created that receives an absorber coming in with an horizontal orientation. At the end of the transport device 20, an absorber will be transferred in a horizontal position again. The transport device will be used as a delay/storage step after the absorbers are coated, and may provide a delay/storage during drying of a coating layer.

In FIGS. 9 and 10, details of an embodiment of the inlet and/or outlet are shown. The absorber 1 has an upper metal plate 31 and a lower metal plate 32. For clarity, in FIGS. 1-7, the upper metal plate 31 actually was the lower metal plate that was not visible. The upper metal plate 31 is provided with a through hole 36. Over the through hole 36, a nipple is provided that allows attachment of the adapters discussed earlier for expanding the absorber. The nipple also allows attachment of conduits for providing or removing fluid to or from the absorber. To that end, the nipple can be provided with attachment provisions. Examples of such provisions comprise internal thread, external thread, or external snap-fit provisions. The nipple 33 in this embodiment further comprises a flange 34 for attaching the nipple 33 to the upper metal plate 31. To that end, in this embodiment it is provided with a circumferential weld or solder zone 35. Alternatively, if the nipple is soldered, no flange may be needed.

In FIG. 10, a cross section of the inlet/outlet is shown, showing a particular attachment provision that is used for attaching the nipple 33 to the upper metal plate 31. At the location of through hole 36, a layer 37 is provided that prevents the upper metal plate 31 and the lower metal plate 32 to become attached to one another when the flange 34 of nipple 33 is welded or soldered to the upper metal plate 31 via zone 35. In production, first the upper metal plate 31 is provided with through hole 36. Next, at the locations of the through holes in the upper plate 31, the lower metal plate 32 is provided with the attachment-preventing layer 37. Next, the upper metal plate 31 is placed on top of the lower metal plate 32. Next, it can be selected to first weld the metal plates 31 and 32 onto one another and then weld or solder the nipple 33, or to first weld or solder the nipple 33 and then weld the metal plate onto one another.

The layer 37 can be a stick-preventing layer known in the art. For instance, a carbon layer can be used, or alternatively a coating of film that prevents the plates 31 and 32 to be welded together at the inlet and outlet.

In an embodiment, instead of the serpentining fluid channel, other layouts may be considered. In particular, it was found that an inward-spiralling channel having its inlet at or near the rim and its outlet at or near the centre has advantages. In such an embodiment, the rim or edge of the absorber stay relatively cool. When mounting the absorber with placement holders at the rim, it is evident that simple polymer holders will suffice. In the inward-spiralling channel layout, in an embodiment the fluid to be used for heat transfer is glycol.

Before entering the steps illustrated above, the metal plates 31 and 32 are placed on top of one another and welded together. It was found that laser welding provides an excellent technique to make a fluid-tight weld and with a quality to last many temperature cycles during the lifetime of the absorber. In fact, in an embodiment, continuous welds are produced using spots that at least partially overlap. It was found that using elongated spots that partially overlap, a fluid-tight weld could be produced. For the solar heat absorber, a continuous, circumferential weld was made near the circumference of the metal plates 31 and 32. This weld encloses both the inlet and outlet. Furthermore, one or more welds are provided that define (alter the absorber is expanded) one or more fluid channels. The fluid channel of channels connect the inlet(s) and outlet(s). In FIGS. 11-13, some layouts of the welds and positioning of inlet and outlet are shown.

In FIG. 11, the fluid channel has a zigzag layout that was already discussed above. The metal plates 31, 32 are placed on top of one another and pressed together. Next, the metal plates 31, 32 are welded together at several points. A circumferential weld 40 is provided which encloses an inlet and outlet opening. The circumferential weld 40 is made close to the rim. The circumferential weld in fact defines the circumference of the solar heat absorber.

Furthermore, continuous welding lines 41 are provided that define the sides of the fluid channel. In this embodiment, a single fluid channel runs from the left upper corner, serpentines and ends in the right lower corner of the solar heat absorber. To that end, alternately one welding line 41 starts from the circumferential weld 40 and runs from one starting side towards an opposite side, ending at a distance from the circumferential weld 40, and a next welding line 41 starts from the circumferential weld 40 at the opposite side and runs from the opposite side towards the starting side, ending at a distance from the circumferential weld 40 at the starting side.

Spaced away from the fluid channel-defining welding lines 41, spot welds 44 are provided. After expansion of the fluid channel, the metal plates 31, 32 will remain attached to one another, at those spot welds 44. In this way, in the fluid channel obstructions are created that have several functions. First, they provide turbulence members, for assuring that a better heat transfer between the metal plates 31, 32 and the fluid running in the fluid channel. Furthermore, these spot welds 44 provide additional rigidity to the solar heat absorber. The cross section of the spots can in an embodiment be between 5-15 mm. Usually these spot welds 44 are circular.

In FIG. 12, the fluid channel has a spiral shape. In this embodiment, again the circumferential weld 40 is provided, and an inlet 42 and an outlet 43 for fluid. Furthermore, an inwards-spiralling continuous weld 41 is provided for defining the fluid channel. In this channel layout, the circumference of the solar heat absorber remains relatively cool, and the centre will be heated. In this way, the solar heat absorber van he clamped along the edge or rim of the solar heat absorber which puts less constraints on a clamping system for holding the solar heat absorber fixed to a building.

In the embodiment of the inward spiralling fluid channel, one single, continuous, spiralling weld 41 would be sufficient. Again, the width of such a weld at the interface between the upper and lower plate would be between 0.3-0.8 mm, depending on the thickness of the upper and lower plate. In this drawing, the spot welds 44 in the fluid channel as indicated in FIG. 11 are not shown, but can also be present and provide the advantages already discussed.

In FIG. 13, the solar heat absorber has a fluid channel which has a substantially zigzag layout but laid out in such a way that fluid inlet 42 and fluid outlet 43 are provided near the same corner of the solar heat absorber. This makes installation easier.

The solar heat absorber again has a continuous, circumferential weld 40. When providing the circumferential weld 40 a little remote from the edge as indicated in this drawing, although not on scale, after expanding the fluid channel a rim is created which functions as an attachment rim for fixing the solar heat absorber in a housing.

It is to be understood that the above description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention. The scope of the invention is to be limited only by the following claims. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the spirit and scope of the present invention. 

What is claimed is:
 1. A method for producing an absorber for a solar collector, the absorber having a fluid inlet and a fluid outlet, the method comprising: providing a first substantially flat metal plate and a second substantially flat metal plate; laser welding the first substantially flat metal plate and the second substantially flat metal plate together at least along a perimeter of the first substantially flat metal plate and the second substantially flat metal plate via at least a circumferential weld, while pressing together the first substantially flat metal plate and the second substantially flat metal plate; placing the welded first substantially flat metal plate and the welded second substantially flat metal plate in a press and applying pressure at the welds using a pressure of at least 200 tons per m² of the absorber; providing a fluid under a pressure to the absorber via at least one of the fluid inlet and the fluid outlet, the fluid being applied at a pressure difference of at least 20 bar between the fluid inlet and the fluid outlet to thereby inflate the absorber, expanding the welded first substantially flat metal plate and the welded second substantially flat metal plate to thereby create a permanent fluid channel between the first substantially flat metal plate and the second substantially flat metal plate and also fluidically-couple the fluid inlet to the fluid outlet.
 2. The method of claim 1, further comprising providing at least one through hole in one of the first substantially flat metal plate and the second substantially flat metal plate to thereby provide the fluid inlet, and at least one through hole in one of the first substantially flat metal plate and the second substantially flat metal plate to thereby provide the fluid outlet.
 3. The method of claim 1, further comprising pressing the first substantially flat metal plate and the second substantially flat metal plate on top of one another along at least a perimeter of the first substantially flat metal plate and the second substantially flat metal plate.
 4. The method of claim 1, wherein the press comprises a die having a pattern which has the shape of the weld and which extends 1-20 mm from the basic surface of the die.
 5. The method of claim 4, wherein the press comprises a first die having the pattern the shape of the weld, and a second, opposite die that has the shape of the weld such that when pressing the welded metal plates, both patterns press the weld between them.
 6. The method of claim 1, wherein the press comprises a die having a pattern which has the shape of the weld and which extends 1-5 mm from the basic surface of the die.
 7. The method of claim 6, wherein the press comprises a first die having the pattern the shape of the weld, and a second, opposite die that also has the shape of the weld such that when pressing the welded metal plates, both patterns press the weld between them.
 8. The method of claim 1, wherein the welding of the first substantially flat metal plate and the second substantially flat metal plate includes proving spot welds in the fluid channel for welding together the metal plates in the fluid channel to thereby provide flow turbulence means between the inlet and the outlet.
 9. The method of claim 1, wherein the weld comprises a circumferential weld enclosing at least the inlet and the outlet.
 10. The method of claim 9, further comprising providing one or more welds that provide boundaries of one or more zigzag fluid channels fluid coupling the inlet and the outlet.
 11. The method of claim 9, further comprising providing in an alternating manner first and second welds such that the first welds run from the circumferential weld at one first end of the absorber end and that end before the circumferential weld at an opposite second end of the absorber, and the second welds run from the circumferential weld at the opposite, second end and that end before the circumferential weld at the first end of the solar heat absorber.
 12. The method of claim 1, wherein spot welds are welded within a circumference of the circumferential weld.
 13. The method of claim 1, wherein the circumferential weld is provided to enclose at least the fluid inlet and the fluid outlet, and one or more welds are provided that provide boundaries of one or more fluid channels between the fluid inlet and the fluid outlet, the welds providing a fluid channel spiralling from a side of the absorber to a centre of the absorber.
 14. The method of claim 1, wherein before the first substantially flat metal plate and the second substantially flat metal plate are placed on top of one another, weld-preventing layers are provided between the first substantially flat metal plate and the second substantially flat metal plate at the positions of the fluid inlet and the fluid outlet.
 15. The method of claim 1, wherein at the location of one of the inlet and the outlet, one of the first substantially flat metal plate and the second substantially flat metal plate is provided with a through hole.
 16. The method of claim 1, wherein before the first substantially flat metal plate and the second substantially flat metal plate are expanded, nipples provided with a flange are provided at the through holes and welded to one of the first substantially flat metal plate and the second substantially flat metal plate with the flange around the through holes before the fluid under pressure is provided to the absorber.
 17. The method of claim 1, wherein before the first substantially flat metal plate and the second substantially flat metal plate are expanded, nipples are provided at the through holes and attached to one of the first substantially flat metal plate and the second substantially flat metal plate and around the through holes before the fluid under pressure is provided to the absorber without attaching together the first substantially flat metal plate and the second substantially flat metal plate at the locations of the through holes.
 18. The method of claim 1, further comprising coating the solar collectors after welding, and transporting the coated solar collectors after coating using a transport system having a buffer sized to retain the coated solar collectors at a time sufficient to permit the coating to dry.
 19. A solar collector comprising: a solar heat absorber including a fluid inlet and a fluid outlet; a first substantially flat metal plate and a second substantially flat metal plate being laser welded to the first substantially flat metal plate at least along a perimeter of the first substantially flat metal plate and the second substantially flat metal plate, and including at least a circumferential weld; a fluid channel provided between the first substantially flat metal plate and the second substantially flat metal plate; and wherein the fluid inlet is fluidically coupled to the fluid outlet.
 20. A solar collector comprising: a solar heat absorber including a first metal plate and a second metal plate welded together at respective perimeters thereof via a continuous, circumferential weld, a fluid inlet and a fluid outlet provided within the circumferential weld, at least one continuous weld extending between the fluid inlet and the fluid outlet, and a plurality of spot welds within the circumferential weld. 